Health Canada has issued a warning after face masks issued to schools and daycares in Quebec were found to contain a toxic chemical.

On Friday, Quebec sent out a directive to institutions to stop using SNN200642 face masks made by Metallifer as they contain “a potential for early lung toxicity.”

“If you have this type of mask in stock, we ask that you stop distributing them and store them in a secure place right away,” the directive reads.

The masks contain “nanoform graphene,” a thin coating of carbon which Health Canada has discovered to be potentially toxic if inhaled.

The masks were also distributed to staff at Revenue Quebec and the Montreal public transport system. The masks in question were all adult size, blue on one side and grey on the other.

The Société de Transport de Montréal (STM) told CTV the masks were removed on Friday immediately after Health Canada issued the safety warning. Staff had been using the masks for several months.

“The health of our employees is a priority, and if we had heard of something we would have been investigating, but at the time that we acquired the masks they were in compliance with the regulations,” spokesman Philippe Dery said.

“As soon as we got the preventive recall note we reacted promptly.”

Staff in Montreal schools and the STM have previously complained about the masks, including reports of rashes and difficulty swallowing.